Leucite glass-ceramics are extensively used in Dentistry to produce Dental prostheses including: Dental crowns, bridges, inlays, and veneers. These materials are particularly useful as they are biocompatible, highly aesthetic and can be fused, extruded, or machined into Dental restorations with ease. Such materials have also traditionally been fused to metal substrates and provided good clinical longevity over about 10 to 12 years.
There is an increased demand for all-ceramic restorations, which may increase further with the ageing population and the ever increasing use of adhesive dentistry.
However, all-ceramic restorations are still susceptible to brittle fracture (Kelly et al. J Prosthet Dent, 1996; 75:18-32), due to the presence of complex stress patterns during function and in areas of reduced ceramic thickness and being in a moist environment (Jones. Biocompatibility of Dental Materials, CRC Press Inc, 1982:81-96). Clinical failure rates for all-ceramic crowns are 52% (Burke and Lucarotti. J Dent, 2009, 37:12-24), and for porcelain veneers, failure rates are 47% (Burke and Lucarotti. J Dent, 2009, 37:31-38) over 10 years placed in the General Dental Service (cost of veneers=£6.2 million). This is unacceptably high and needs to be drastically improved.
Survival rates for leucite glass-ceramics are improved at 84.4% for posterior crowns after 11 years (Fradeani and Redemagni. Quintessence Int. 2002; 33:503-510).
Ceramic restorations when fused to metals have a lower failure rate of 38% Burke and Lucarotti. J Dent, 2009, 37:12-24) due to the reinforcement of the weak ceramic. Reinforcing weak porcelains with high strength metals or ceramic cores (zirconia) currently comes at high manufacturing cost and requires gross tooth reduction. Newer high strength zirconia and alumina core materials have been released on the market to address the all-ceramic strength issues. The opacity/hardness of these materials requires that they are veneered with aesthetic thermally compatible glasses which exhibit lower strengths than current leucite glass-ceramic materials. Failure of these materials at the core-veneer interface has subsequently been reported (Taskonak et al. Dent Mater, 2008; 24:1077-1082). Issues with the etching and long term adhesive bonding of fully crystalline zirconia core ceramics may also limit their applications (Walker et al., Dent Mater 2003; 19: 645-652). An overall thickness of 1.5-2 mm must be removed from the tooth surface in order to accommodate these bi- or tri-layer structures, which does not encourage the conservation of tooth structure.
The present development of leucite glass-ceramics which have a high area fraction of leucite fibers/particles and can combine high flexural strength together with good aesthetics is therefore useful in the field of Dentistry. These materials can be easily etched and adhesively bonded and used in situations of minimal tooth reduction.
Leucite glass-ceramics are typically produced via nucleation and crystal growth heat treatments of a glass. Typical commercial production methods produce dental glass-ceramics with 17 to 45% leucite content (Piche' et al., J Biomed Mater Res 1994; 28:603-9. Mackert et al., Int J Prosthodont, 1996; 9:261-265) with large leucite crystal sizes (˜10 μm) and more irregular morphologies (Cattell et al., J Dent 1999; 27:183-96). The thermal expansion mismatch between the tetragonal leucite crystals and the glass matrix developed during leucite transformation can often cause signs of microcracking around larger non-uniform leucite crystals (Mackert et al., J Dent Res 1996; 75:1484-90), that has been linked to reduced mechanical properties (Shareef et al., 3 Mater Sci: Materials in medicine, 1994; 5:113-118. Cattell et al., Dent Mater 2001; 17:21-33). Current leucite glass-ceramic materials have low flexural strengths (120-140 MPa) and many of these materials have been linked to microstructural failure as reported in the literature. These materials can also have coarse microstructures that can wear the opposing teeth (Oh et al., J Prosthet Dent, 2002, 87:451-9.5, 6). This is another concern, as many patients grind their teeth at night which causes these materials to be more destructive to the opposing tooth surfaces.
The present invention seeks to design glasses which are thermally matched with the leucite crystal phase, preventing any glass matrix micro-cracking. This has led to a significant increase in the flexural strength (212-235 MPa) of these new materials compared to commercial materials. The refractive index of the glass and crystal phase is similarly matched in these new materials ensuring no loss of translucency. A high area fraction (>65%) of leucite crystals can therefore be produced and the morphologies controlled to give densely dispersed areas of orientated fibers, spheres, and rosette-shaped domains. In the present invention, solid state nuclear magnetic resonance spectroscopy has been used to find the optimal ratio between the glass and ceramic phase that delivers the highest strength of these new materials.
US 2005/0034631 A1 describes a high strength leucite glass-ceramic. However, this product is doped with zirconium oxide (60% by weight) and without such doping, the strength is only 107 MPa. Chemical curing is also employed to yield high strengths. These glasses also contain titanium dioxide in the base glass. The use of zirconium oxide is problematic in that it can cause opacity and may affect the translucency and aesthetics of the dental product. Given the high volume fractions of zirconium oxide present in the ceramics described in this publication, wear properties would be compromised since zirconia ceramics are known to suffer from low temperature degradation (J. R Kelly et al., Dent Mater 2008; 24:289-298). The use of chemical curing to achieve high strengths also adds to the manufacturing complexity and cost, and therefore should be avoided.
A high strength glass-ceramic described in WO 2009/073079 A1/WO 2009/038800 A1 benefits from a ball milling technique to produce the surface crystallization of leucite glass-ceramics. Without the ball milling of the glass powder, the biaxial flexural strength of this leucite glass-ceramic is only 153 MPa. This lengthy (4 hrs) ball milling procedure is problematic since it can induce zirconia contamination and additionally increases manufacturing time and cost. Furthermore, the process for producing the glass-ceramics of WO 2009/073079 A1/WO 2009/038800 A1 utilises a surface crystallization method, whereas it would be advantageous if leucite glass-ceramics were developed that could be crystallized by both a bulk and surface method. At present, bulk crystallization is rarely, if ever, reported for leucite glass-ceramics. In addition, the glass-ceramics of WO 2009/073079 A1/WO 2009/038800 A1 exclusively consist of round, ellipsoidal, crystals, whereas it would be advantageous if leucite glass-ceramics were developed that comprise a range of controllable morphologies including: leucite fibers, rosettes, and spheres. It would also be advantageous if the aspect ratio of the crystals could be varied in the said glass-ceramic via compositional changes.
U.S. Pat. No. 6,527,846 describes a high volume fraction leucite glass-ceramic with leucite crystals in needle or rod form with high flexural strength (200 MPa). No information is provided as to how to control the size of these crystals, or how to effect changes to the morphology by formulation or processing. It would be advantageous if leucite glass-ceramics were developed in which the morphology, area fraction, and refractive index could be controlled in order to influence the strength, thermal expansion, and aesthetics of the finished Dental restoration.